The invention relates to a system for digital information transmission, to a method and to a device for producing a transmission signal, to a demodulation method and to a receiver for receiving a transmission signal.
A multicarrier transmission method for transmitting digital data is generally known in information technology, which method uses a discrete Fourier transform for signal synthesis. Multicarrier transmission methods are described, for example, by John Bingham in xe2x80x9cMulticarrier Modulation for Data Transmission: An Idea Whose Time Has Come,xe2x80x9d IEEE Communications Magazine, May 1990, pages 5-14.
One specific method in this context is, for example, the so-called OFDM method (OFDM=orthogonal frequency division multiplexing), also referred to as the DMT method (DMT=discrete multitone), which is described by Weinstein and Ebert in xe2x80x9cData Transmission By Frequency-Division Multiplexing Using The Discrete Fourier Transform,xe2x80x9d IEEE Transactions on Communication Technology, vol. COM-19, no. 5, October 1971, pages 628-34. The action of pulse interference between successive symbols is solved using the already known method of inserting a guard interval.
U.S. Pat. No. 5,345,439 to Marston discloses a signal processing apparatus, in which one appliance uses various modulation formats for universal application. U.S. Pat. No. 4,313,197 to Maxemchuk discloses a device for multiplexing and demultiplexing signals, in particular voice signals. Fourier transformation is used in that case. One application of the OFDM method for optical transmission is known from Olofsson et al.: xe2x80x9cDesign of OFDM Systems at High Power Levels,xe2x80x9d 17 Journal of Optical Communications, 1996, 3, pages 95-97.
German Patent Application No. 43 10 031 discloses a method for correcting the phase amplitude of a broadband received signal, in which the individual carrier frequencies that are used are phase modulated using the COFDM method. In that method, the data blocks are synchronized to one another in the time domain.
Fundamental principles relating to these techniques and methods can be found in the textbook xe2x80x9cNachrichtenubertragungxe2x80x9d [Information transmission] by Karl Dirk Kammermeyer, Teubner Verlag, Stuttgart, 1992, in particular on pages 68-70, 372, 378, 379 and 606-613.
The most recent advance in the art is described in the international PCT publication WO 91/14316. Of similar interest is a published article by Paul Moose: xe2x80x9cDifferentially Coded Multi-Frequency Modulation For Digital Communications,xe2x80x9d FIFTH EUROPEAN SIGNAL PROCESSING CONFERENCE, Sep. 18-21, 1990, Amsterdam, Netherlands, pages 1807-10. These publications provide so-called baud synchronization, in which a number of blocks of code symbols are preceded by a common synchronization baud.
Error coding for the so-called PSK method with an RF modem is known from Gene Porter: xe2x80x9cError Distribution And Diversity Performance Of A Frequency-Differential PSK HF Modem,xe2x80x9d IEEE TRANSACTIONS ON COMMUNICATION TECHNOLOGY, Vol. 16, No. 4, August 1996, New York, pages 567-75.
A further method and arrangement for differential modulation of signals in a multichannel transmission system is known from international PCT publication WO 92/20179.
It is accordingly an object of the invention to provide a system with associated methods and devices for digital information transmission for channels with dispersive distortion, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which avoids complex carrier phase and sampling phase synchronization, with high bandwidth efficiency and a low error rate.
With the foregoing and other objects in view there is provided, in accordance with the invention, an improved method of producing a transmission signal for digital information transmission, wherein, for multicarrier transmission, subcarriers are modulated with channel coded parallel code symbols of an input signal by means of differential coding in the frequency domain, and synchronization information is added to the transmission signal. The improvement comprises:
prior to the differential coding, error coding and/or interleaving;
after the differential coding, producing a serial output signal by means of parallel/serial conversion;
assigning each block of code symbols, as synchronization information, a preamble for block synchronization; and
adding to each block of code symbols a transmission pause after the differential coding.
In accordance with an added feature of the invention, the serial output signal is subjected to digital/analog conversion.
In accordance with an additional feature of the invention, the serial output signal is transmitted in transmission blocks transmitted with time discontinuities and/or in burst mode.
In accordance with another feature of the invention, after the differential coding, there is added to each block of code symbols at a start thereof, a periodic continuation as a guard interval.
In accordance with a further feature of the invention, prior to the differential coding, the code symbols are assigned to a signal space by means of a PSK or QAM method.
In accordance with again a further feature of the invention, after the differential coding, the signal is subjected to an inverse discrete Fourier transform (IDFT).
With the above and other objects in view there is also provided, in accordance with the invention, a device for producing a transmission signal for digital information transmission with a multicarrier transmission method, comprising:
a module for carrier modulation receiving channel coded, parallel code symbols of an input signal and adding code symbols to subcarriers by means of differential coding in a frequency domain, wherein the transmission signal contains synchronization information;
a module for at least one of error coding and interleaving connected upstream of the module for carrier modulation in a signal flow direction;
a parallel/serial converter connected downstream of the module for carrier modulation in the signal flow direction;
a synchronization device for producing synchronization information, the synchronization device producing a preamble for each produced block of code symbols for block synchronization; and
an element for producing a transmission pause between successive blocks.
There is also provided, in accordance with the invention, an n improved demodulation method for a received multicarrier transmission signal, wherein code symbols are modulated onto subcarriers by means of differential coding in the frequency domain, and wherein blocks follow one another with time discontinuities, and a phase of complex subcarrier coefficients is distinguished and blocks of code symbols are synchronized. The improvement comprises the following steps:
prior to differentiation, subjecting the received transmission signal to analog/digital conversion with subsequent serial/parallel conversion;
prior to differentiation, removing any transmission pause between adjacent blocks;
subsequent to differentiation, carrying out one of error detecting respective preambles assigned to the blocks, and synchronizing each of the blocks.
There is furthermore provided, in accordance with the invention, a receiving device for a received multicarrier transmission signal, wherein code symbols are modulated onto subcarriers by means of differential coding in a frequency domain, and wherein blocks follow one another with time discontinuities, comprising:
an analog/digital converter receiving a transmission signal;
a serial/parallel converter connected downstream of the analog/digital converter in a signal flow direction;
a demodulator for differentiating a phase of complex subcarrier coefficients and for synchronizing blocks connected downstream of the serial/parallel converter in the signal flow direction;
an error decoding and/or deinterleaving module connected downstream of the demodulator in the signal flow direction;
means for removing a transmission pause between the blocks; and
a synchronization device for detecting preambles assigned to the respective blocks of code symbols and for synchronizing with the preambles.
With the above and other objects in view there is also provided, in accordance with the invention, a system for digital information transmission with a multicarrier transmission method, comprising:
a device for producing a transmission signal, the device having a module for carrier modulation receiving channel coded parallel code symbols of an input signal and adding to subcarriers thereof the code symbols by means of differential coding in a frequency domain, an error coding and/or interleaving module connected upstream of the module for carrier modulation in a signal flow direction, and a parallel/serial converter connected downstream of the module for carrier modulation in the signal flow direction;
means for producing synchronization information by producing a preamble for each produced block of code symbols for block synchronization, and an element for inserting a transmission pause between successive blocks;
a receiving device for the received multicarrier transmission signal in which the blocks follow one another, possibly with time discontinuities, the receiving device having a serial/parallel converter connected downstream of the parallel/serial converter in the signal flow direction, a demodulator for differentiating a phase of complex subcarrier coefficients connected downstream of the serial/parallel converter, an error decoding and/or deinterleaving module connected downstream of the demodulator, means for removing the transmission pause from between the blocks, means for detecting the preambles respectively assigned to respective blocks, and for synchronizing the respective blocks.
In order to avoid complex methods for carrier phase and sampling phase synchronization, an essential fundamental principle of the invention is the assignment of the binary code symbols to the subcarriers by differential coding in the direction of the subcarriers, that is to say in the frequency domain. In contrast to this, in the prior art, this is achieved by differential modulation in the time domain. Together with channel coding, and possibly code symbol scrambling along the subcarriers, it is possible to dispense entirely with carrier phase synchronization. The permissible tolerances in this case are particularly high for sampling phase synchronization.
A procedure is provided for block or frame synchronization, which operates with a preamble in order to identify the block start, and thus the sampling phase. An autocorrelation of the Barker type, for example, is suitable for this purpose. This is advantageous in the case of an application with discontinuities during operation, particularly in a cable-based network, such as the power distribution network, or in a radio network.
The information transmission method achieved using the invention is suitable for time-variant or time-invariant channels with intersymbol interference (reflections), that is to say with dispersive distortion. In this case, the available frequency band is divided into M sub-channels, in which case all the sub-channels are independent of one another and have a response which has virtually no frequency selectivity. The response without any frequency selectivity results from the fact that the bandwidth of a sub-channel is so narrow that the channel transfer function within a sub-channel is virtually constant. The invention is suitable, for example, for use with remote data transmission for meters, or in wire-free telecommunications or telephone technology. The following text explains the solutions to the problem and advantageous refinements in more detail.
A method for producing a transmission signal for digital information transmission provides that subcarriers are modulated with channel-coded parallel code symbols of an input signal by means of differential coding in the frequency domain. The code symbols can advantageously be assigned, before the differential coding, to a signal space using the PSK or QAM method, which technique is also called mapping. The code symbols can in this case be error-coded and/or interleaved before the differential coding. This allows a high Hamming spacing and low error rate or error probability to be achieved.
It is advantageous if each block (which can also be described as a time domain sequence) of code symbols has added to it, after the differential coding, a transmission pause and, possibly, a periodic continuation in the sense of a preamble, in particular at the block start. This simplifies demodulation and block synchronization in the receiver. The transmission pause avoids a DMT block being interfered with by the influence of an adjacent block, thus simplifying demodulation in the receiver.
For direct, simple production of the transmission signal, a serial output signal can be produced after the differential coding by means of parallel/serial conversion, which output signal is advantageously subsequently subjected to digital/analog conversion, by means of which it is possible to change to a signal that is continuous in the time domain. The output signal may in this case comprise serial transmission blocks, which are transmitted with time discontinuities, in particular using the burst mode. An inverse discrete Fourier transform is preferably carried out for differential coding.
With regard to the device for producing a transmission signal for digital information transmission, the invention provides that this device has a module for carrier modulation, to which channel-coded, parallel code symbols of an input signal are supplied and in which the subcarriers have the code symbols added to them, by means of differential coding in the frequency domain.
A mapping module can advantageously be connected upstream of the module for carrier modulation, in which mapping module the code symbols are assigned to a signal space, using the PSK or QAM method. In other words, the code symbols are assigned a sequence of amplitude coefficients of a PSK constellation, which are subsequently used for modulation of the subcarriers by means of differential coding. An error-coding and/or interleaving module can advantageously be connected upstream of the module for carrier modulation, which results in the error rate during transmission being reduced.
A parallel/serial converter and a digital/analog converter can be connected downstream of the module for carrier modulation, in which case transmission blocks can be produced with time discontinuities, in particular using the burst mode, that is to say not continuously, as the output signal at the output of the digital/analog converter. The module for carrier modulation for the differential coding advantageously comprises an element for inverse discrete Fourier transform. This allows rapid, precise coding.
The solutions according to the invention for the demodulation method at the receiver end and for the demodulator are designed in a corresponding manner to the above transmitter-end refinements, essentially using inverse functions and procedures. In this case, it is advantageous if any transmission pause which is contained in the transmission signal is removed before the process of distinguishing the received transmission signal (also called the received signal). A preamble which is contained in the transmission signal is advantageously detected for block or phase synchronization. The block start can be identified in a simple manner by correlation calculation, calculating the gradient of the correlation magnitudes and threshold-value distinction.
As explained above, the system for digital information transmission includes:
a device for producing a transmission signal with a module for carrier modulation, to which channel-coded parallel code symbols of an input signal are supplied and in which the subcarriers have the code symbols added to them by means of differential coding in the frequency domain, and
a receiving device with a demodulator, in which the phase of the complex subcarrier coefficients is distinguished.
The transmission path used may be cable-free or wire-free, for example a radio link. An optical carrier medium, in particular an optical conductor, is also suitable for use as the transmission path. A power cable, a non-power cable or a corresponding network can also be used in a simple way as the transmission path. A link similar to an EIB bus or a power distribution network is suitable, for example, for this purpose. One preferred field of application for the invention is remote meter reading in an electrical distribution network.
It is essential for the preferred embodiment
that parallel, independent sub-channels are provided at the transmitter end by using inverse discrete Fourier transform;
that pulse interference to successive blocks is eliminated by using a periodic continuation;
that discrete Fourier transform of a block at the receiver end and decoding of all the sub-channel signals are used to obtain the digital data;
whereby
the digital data are channel-coded;
the modulation of the subcarriers is carried out differentially and in coded form in the direction of the subcarriers; and
at the receiver end, the phase of the complex subcarrier coefficients is distinguished, in such a way that the reception points in the signal space are recovered.
The above concept is characterized by a specific carrier assignment in conjunction with channel coding and so-called interleaving, thus avoiding the need for channel equalization using       1          H      ⁢              (        f        )              ,
and with there being no need for carrier phase synchronization. Furthermore, the requirements for sampling phase synchronization are so minor that the influence of tolerances up to several hundred percent is virtually irrelevant.
The system is preferably produced, with its devices, using computers, processors and/or signal processors with digital information processing and other suitable means for digital signal processing. The methods can at least partially be implemented in this case in the form of programs or else in an ASIC.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a system for digital information transmission, with associated methods and devices, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.